I'm thinking perhaps we can just do this for now:
cyrus_require_provisioning = "internal_plain"
And mod_auth_cyrus then loads mod_auth_internal_plain and talks with that.
Stacked auth providers is something we want, but not in 0.9.

This is as important as ever, but we don't have dev resources to work on it without delaying the 0.9 release further. At the same time, it can be done fairly simply through module changes after the release if necessary.
Removing the 0.9 milestone